A number of degenerative diseases are thought to be caused by or be associated with alterations in mitochondrial metabolism. These include Alzheimer's Disease, Parkinson's Disease, Huntington's disease, dystonia, Leber's hereditary optic neuropathy, schizophrenia, and myodegenerative disorders such as "mitochondrial encephalopathy, lactic acidosis, and stroke" (MELAS), and "myoclonic epilepsy ragged red fiber syndrome" (MERRF).
Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is characterized by loss and/or atrophy of neurons in discrete regions of the brain, and that is accompanied by extracellular deposits of .beta.-amyloid and the intracellular accumulation of neurofibrillary tangles. It is a uniquely human disease, affecting over 13 million people worldwide. It is also a uniquely tragic disease. Many individuals who have lived normal, productive lives are slowly stricken with AD as they grow older, and the disease gradually robs them of their memory and other mental faculties. Eventually, they cease to recognize family and loved ones, and they often require continuous care until their eventual death.
Individuals who are afflicted with AD may have one of two forms of this disease: "familial" AD or "sporadic" AD. Familial AD has an early onset, usually beginning in the forties or fifties. As the name suggests, the occurrence of this form of AD follows conventional patterns of Mendelian inheritance. Sporadic AD, which is believed to account for 90-95% of all cases of AD, is a late-onset disease which is not inherited in Mendelian fashion, and it thus does not appear to be caused by nuclear chromosomal abnormalities.
There is evidence that defects in oxidative phosphorylation are at least a partial cause of sporadic AD. The enzyme cytochrome C oxidase (COX), which makes up part of the mitochondrial electron transport chain (ETC), is present in normal amounts in AD patients; however, the catalytic activity of the enzyme in AD patients and in the brains of AD patients at autopsy has been found to be abnormally low. This suggests that the genes for COX in AD patients are defective, leading to decreased catalytic activity that in some fashion causes or contributes to the symptoms that are characteristic of AD.
COX in humans and other mammals, is composed of at least 13 subunits. At least ten of these subunits are encoded by nuclear genes; the remaining three subunits (COX I, II and III) are encoded by mitochondrial genes. The catalytic centers of COX are associated with COX I and COX II. Thus, catalysis by COX is dependent upon the proper function of two of the subunits that are encoded for by the mitochondrial DNA (mtDNA).
Specific point mutations in the mtDNA genes that encode for COX subunits I and II segregate with AD, and are rarely found in age-matched controls or patients with other neurological disorders. These specific AD-associated mtDNA point mutations result in alterations in the primary structure of the encoded proteins, and these might be expected to perturb COX catalytic activity by distorting the secondary and/or tertiary structures of the COX complex. The reduced COX activity caused by these defects could lead to increased intracellular levels of oxygen free radicals; and the cumulative effects of free radical-mediated lipid oxidation ultimately cause the degenerative neurological changes that are characteristic of AD (1).
Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by the loss and/or atrophy of dopamine-containing neurons in the pars compacta of the substantia nigra of the brain. Like AD, PD also afflicts the elderly. It is characterized by bradykinesia (slow movement), rigidity and a resting tremor. Although L-Dopa treatment reduces tremors in most patients for a while, ultimately the tremors become more and more uncontrollable, making it difficult or impossible for patients to even feed themselves or meet their own basic hygiene needs.
Like sporadic AD, most cases of PD appear sporadically in the population; even with identical twins, one may have the disease, and the other not. This suggests that nuclear chromosomal abnormalities are not the cause of this disease. Furthermore, it has been shown that the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces parkinsonism in animals and man. MPTP is converted to its active metabolite, MPP+, in dopamine neurons; it then becomes concentrated in the mitochondria. The MPP+ then selectively inhibits the enzyme NADH:UBIQUINONE OXIDOREDUCTASE ("Complex I"), leading to the increased production of free radicals, reduced production of adenosine triphosphate, and ultimately, the death of affected dopamine neurons.
Complex I is composed of 40-50 subunits; most are encoded by the nuclear genome and seven by the mitochondrial genome. Since parkinsonism may be induced by exposure to mitochondrial toxins that affect Complex I activity, it appears likely that defects in the mitochondrial genes that encode Complex I proteins may contribute to the pathogenesis of PD by causing a similar biochemical deficiency in Complex I activity. Indeed, defects in mitochondrial Complex I activity have been reported in the blood and brain of PD patients (2).
Similar theories have been advanced for similar relationships between mtDNA mutations and other neurological diseases, including Leber's hereditary optic neuropathy, schizophrenia, "mitochondrial encephalopathy, lactic acidosis, and stroke" (MELAS), and "myoclonic epilepsy ragged red fiber syndrome" (MERRF).
The identification of therapeutic regimens or drugs that are useful in the treatment of disorders associated with such mitochondrial defects has historically been hampered by the lack of reliable model systems that could be used in rapid and informative screening. Animal models do not exist for many of the human diseases that are associated with mitochondrial gene defects. In addition, appropriate cell culture model systems are either not available, or are difficult to establish and maintain. Furthermore, even when cell culture models are available, it is often not possible to discern whether the mitochondrial or the cellular genome is responsible for given phenotype, as mitochondrial functions are often encoded by both genomic and mitochondrial genes. It is therefore also not possible to tell whether the apparent effect of a given drug or treatment operates at the level of the mitochondrial genome or elsewhere.
One approach that has been used to attempt to discern which genome is responsible is to destroy the mitochondrial DNA in cultured cells known to have proper mitochondrial function, and then transfer to such cells the mitochondria from diseased cells (3). However, the resulting cell lines tend to be unstable and hard to culture. Fully differentiated cell lines are used as the targets for transplantation, but their naturally limited life spans makes them particularly unsuitable for screening purposes. In addition, such transformations have not been done using cells of the type that are most affected by the disease, making it unclear whether the mitochondrial deficiencies observed in the transformants are related to the disease state being studied.